As far as I know, there are no null Serial Numbers. However, your reply made me take another look at the query and I should have typed:

Where VehiclesToUpdate.SerialNumber Is Null

When I did that, I returned 157,511 records. I assume these are VINs that are not in the VehiclesToUpdate that should be.

As for stats, there are 209,809 records in the VD table. VehiclesToUpdate returns 177,303 records which leaves 32,506 - NOT 157,511. The next thing I will check is duplicates. That could account for some records, but probably not that many.